Heater and glow plug provided with same

ABSTRACT

A heater includes: a resistor including a heat-generating portion; a lead joined to an end portion of the resistor; and an insulating base covering the resistor and the lead. At a junction between the resistor and the lead, the lead has a shape thicker than the resistor and is connected to the resistor such that the end portion of the resistor is inserted into a front end portion of the lead, a recess is provided on an end surface of the resistor, and a portion of the lead is inserted into the recess.

FIELD OF INVENTION

The present invention relates to a ceramic heater used, for example, as an ignition or flame detection heater for combustion type onboard heating apparatus, an ignition heater for various combustion apparatuses such as kerosene fan heater, a heater for glow plug of automobile engine, a heater for various sensors such as oxygen sensor, or a heater for measuring instrument; and a glow plug provided with the same.

BACKGROUND

A heater used in such applications as glow plug of automobile engine includes a resistor including a heat-generating portion, a lead, and an insulating base. The materials for them are selected and the shapes of them are designed such that the resistance of the lead is lower than that of the resistor.

Here, a junction between the resistor and the lead is a point of change in shape or a point of change in material composition. Thus, there is known a heater in which the interface between the resistor and the lead is tilted when being seen in a cross section parallel to the axial direction of the lead, in order to increase the junction area such that an effect caused by a difference in thermal expansion produced by heat generation or cooling during use is not provided (e.g., see Patent Literature 1 and 2).

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2002-334768

PTL 2: Japanese Unexamined Patent Application Publication No. 2003-22889

SUMMARY Technical Problem

In recent years, temperature rise quicker than that in the related art is desired, and thus the necessity arises to pass a high current through a resistor at start of an engine operation arises. When a high current is passed through a heater and the heater is used as described above, even if the interface between the resistor and the lead is tilted and the junction area therebetween is increased, a problem arises that the difference in thermal expansion between the resistor and the lead is great, thermal stress is concentrated on the junction therebetween (an end portion of the resistor or an end portion of the lead), and a crack is caused therein.

The present invention has been conceived of in view of the above-described problems of the related art, and an object thereof is to provide a highly-reliable and durable heater in which concentration of great thermal stress on a junction between a resistor and a lead is suppressed even when a high current is passed through the resistor in quick temperature rise.

Solution to Problem

A heater according to the present invention is a heater including: an insulating base; a resistor buried in the insulating base; and a lead buried in the insulating base, connected at a front end side thereof to the resistor, and drawn out at a rear end side thereof to a surface of the insulating base. The lead has a shape thicker than the resistor and is connected to the resistor such that an end portion of the resistor is inserted into a front end portion of the lead, a recess is provided on an end surface of the resistor, and a portion of the lead is inserted into the recess.

In addition, it is possible to use the heater according to the present invention as a glow plug including the heater having the above configuration and a metallic retaining member which is electrically connected to the lead and retains the heater.

Advantageous Effects of Invention

According to the heater of the present invention, even when a high current flows in quick temperature rise, it is possible to dissipate heat inside the resistor to the lead having a lower resistance value than that of the resistor. Therefore, it is possible to restrain heat from staying at the junction and to reduce load by heat generation. As a result, even when the temperature is repeatedly increased and decreased, it is possible to suppress occurrence of a crack in the junction. Thus, the reliability and the durability of the heater are improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is an enlarged longitudinal cross-sectional view of a principal part showing an example of an embodiment of a heater according to the present invention, and FIG. 1( b) is a transverse cross-sectional view taken along an X-X line shown in FIG. 1( a).

FIG. 2( a) is an enlarged longitudinal cross-sectional view of a principal part showing another example of the embodiment of the heater according to the present invention, and FIG. 2( b) is a transverse cross-sectional view taken along an X-X line shown in FIG. 2( a).

FIG. 3( a) is an enlarged longitudinal cross-sectional view of a principal part showing still another example of the embodiment of the heater according to the present invention, and FIG. 3( b) is a transverse cross-sectional view taken along an X-X line shown in FIG. 3( a).

FIG. 4( a) is an enlarged longitudinal cross-sectional view of a principal part showing still another example of the embodiment of the heater according to the present invention, and FIG. 4( b) is a transverse cross-sectional view taken along an X-X line shown in FIG. 4( a).

FIGS. 5( a) and 5(b) are each an enlarged longitudinal cross-sectional view of a principal part showing still another example of the embodiment of the heater according to the present invention.

FIG. 6 is a schematic longitudinal cross-sectional view showing an example of an embodiment of a glow plug according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, examples of an embodiment of a heater according to the present invention will be described in detail with reference to the drawings.

FIG. 1( a) is a longitudinal cross-sectional view showing an example of the embodiment of the heater according to the present invention, and FIG. 1( b) is a transverse cross-sectional view taken along an X-X line shown in FIG. 1( a). In addition, FIG. 2( a) is a longitudinal cross-sectional view showing another example of the embodiment of the heater according to the present invention, and FIG. 2( b) is a transverse cross-sectional view taken along an X-X line shown in FIG. 2( a).

The heater 1 of the embodiment includes an insulating base 9, a resistor 3 buried in the insulating base 9, and leads 8 which are buried in the insulating base 9, are connected at front end sides thereof to the resistor 3, and are drawn out at rear end sides thereof to a surface of the insulating base 9. Each lead 8 has a shape thicker than the resistor 3 and is connected to the resistor 3 such that an end portion of the resistor 3 is inserted into the front end portion of the lead 8. Recesses 31 are provided on end surfaces of the resistor 3, and a portion of each lead 8 is inserted into the recess 31.

The insulating base 9 in the heater 1 of the embodiment is formed, for example, in a bar shape. The insulating base 9 covers the resistor 3 and the leads 8. In other words, the resistor 3 and the leads 8 are buried in the insulating base 9. Here, the insulating base 9 is preferably made of ceramics. Thus, the insulating base 9 is able to resist higher temperatures than metals, and hence it is possible to provide a heater 1 having further improved reliability in quick temperature rise. Specific examples thereof include ceramics having electrical insulating properties such as oxide ceramics, nitride ceramics, and carbide ceramics. Particularly, the insulating base 9 is preferably made of silicon nitride ceramics. This is because silicon nitride, which is a principal component, is good in terms of high strength, high toughness, high insulating properties, and heat resistance. It is possible to obtain the silicon nitride ceramics, for example, by mixing 3 to 12% by mass of a rare earth element oxide such as Y₂O₃, Yb₂O₃, or Er₂O₃ as a sintering aid, 0.5 to 3% by mass of Al₂O₃ with silicon nitride as the principal component, further mixing SiO₂ therewith such that an SiO₂ amount contained in a sintered body is 1.5 to 5% by mass, molding the mixture into a predetermined shape, and then conducting firing through hot pressing at 1650 to 1780° C.

In addition, when one made of silicon nitride ceramics is used as the insulating base 9, it is preferred that MoSiO₂, WSi₂, or the like is mixed and dispersed therein. In this case, it is possible to make the coefficient of thermal expansion of the silicon nitride ceramics as the base material to be close to the coefficient of thermal expansion of the resistor 3, and thus it is possible to improve the durability of the heater 1.

When the resistor 3 has a linear shape as shown in FIG. 1, it is possible to make a region between the leads 8 to be a heat-generating portion 4. To selectively make into the heat-generating portion 4, a region in which a cross-sectional area is partially reduced or a region having a helical shape may be provided. In addition, when the resistor 3 has a folded shape as shown in FIG. 2, it is possible to make the region of the resistor 3 between the leads 8 to be the heat-generating portion 4, and a portion around the middle point of the folded portion becomes the heat-generating portion 4 that generates heat most. One containing a carbide, a nitride, a silicide, or the like of W, Mo, Ti, or the like as a principal component may be used as the resistor 3. When the insulating base 9 is the above material, tungsten carbide (WC) among the above-described materials is good as the material of the resistor 3 in that the difference in coefficient of thermal expansion from the insulating base 9 is small, in having a high heat resistance, and in having a low specific resistance. Furthermore, when the insulating base 9 is made of silicon nitride ceramics, the resistor 3 preferably contains, as a principal component, WC which is an inorganic conductor, and the amount of silicon nitride added thereto is preferably equal to or greater than 20% by mass. For example, in the insulating base 9 made of silicon nitride ceramics, tensile stress is generally applied to a conductor component which is to be the resistor 3, since the conductor component has a higher coefficient of thermal expansion than that of silicon nitride. On the other hand, when silicon nitride is added to the resistor 3, it is possible to make the coefficient of thermal expansion of the resistor 3 to be close to the coefficient of thermal expansion of the insulating base 9 and to alleviate stress caused by a difference in coefficient of thermal expansion in temperature rise or temperature fall of the heater 1.

In addition, when the amount of silicon nitride contained in the resistor 3 is equal to or less than 40% by mass, it is possible to make the resistance value of the resistor 3 relatively small and stabilize the resistance value. Therefore, the amount of silicon nitride contained in the resistor 3 is preferably 20% by mass to 40% by mass. More preferably, the amount of silicon nitride is 25% by mass to 35% by mass. Moreover, instead of silicon nitride, boron nitride may be added in an amount of 4% by mass to 12% by mass as a similar additive to the resistor 3.

The thickness of the resistor 3 (the thickness in the up-down direction shown in FIG. 2( b)) is preferably 0.5 mm to 1.5 mm, and the width of the resistor 3 (the width in the horizontal direction shown in FIG. 2( b)) is preferably 0.3 mm to 1.3 mm. By being set within these ranges, it is possible to decrease the resistance value of the resistor 3 and to cause the resistor 3 to sufficiently generate heat. In addition, when the insulating base 9 has a lamination structure formed, for example, by laminating halved molded bodies, it is possible to keep the adhesiveness at the lamination interface of the insulating base 9 having the lamination structure.

One containing a carbide, a nitride, a silicide, or the like of W, Mo, Ti, or the like as a principal component may be used as each lead 8 joined to the end portion of the resistor 3, and an example thereof is one whose resistance value per unit length is made lower than that of the resistor 3 by containing a larger amount of the forming material of the insulating base 9 than that of the resistor 3, or making the cross-sectional area larger than that of the resistor 3.

Each lead 8 may be formed by using the same material as that of the resistor 3. Particularly, WC is preferred as the material of each lead 8 in that the difference in coefficient of thermal expansion from the insulating base 9 is small, in having a high heat resistance, and in having a low specific resistance. In addition, when the insulating base 9 is made of silicon nitride ceramics, each lead 8 preferably contains, as a principal component, WC which is an inorganic conductor, and silicon nitride is preferably added thereto in an amount of equal to or greater than 15% by mass. It is possible to make the coefficient of thermal expansion of each lead 8 to be closer to the coefficient of thermal expansion of the insulating base 9 as the amount of silicon nitride is increased. In addition, when the amount of silicon nitride is equal to or less than 40% by mass, the resistance value of each lead 8 is decreased and stabilized. Therefore, the amount of silicon nitride is preferably 15% by mass to 40% by mass. More preferably, the amount of silicon nitride is 20% by mass to 35% by mass. It should be noted that the resistance value of each lead 8 per unit length may be decreased by making the cross-sectional area of each lead 8 larger than that of the resistor 3, or containing a smaller amount of the forming material of the insulating base 9 than that of the resistor 3.

As shown in FIGS. 1 and 2, each lead 8 has a thicker shape than the resistor 3, and is connected to the resistor 3 such that the end portion of the resistor 3 is inserted into the front end portion of the lead 8. The recesses 31 are provided on the end surfaces of the resistor 3, and a portion of each lead 8 is inserted into the recess 31. In other words, the junction between the resistor 3 and each lead 8 has a configuration in which the end portion of the resistor 3 is inserted into the front end portion of the lead 8 and a portion of the lead 8 is inserted into the recess 31 provided on the end surface of the resistor 3 into which the end portion of the lead 8 is inserted. It should be noted that the junction refers to a region where the interface between the resistor 3 and each lead 8 is present when being seen in a cross section parallel to the axial direction of the lead 8.

Each end portion of the resistor 3 is preferably inserted into the front end portion of each lead 8 by, for example, 0.1 to 1.0 mm. The depth of the recess 31 provided on each end surface of the resistor 3 depends on the amount by which the end portion of the resistor 3 is inserted into the front end portion of the lead 8 but is, for example, 0.01 to 0.3 mm. Examples of the cross-sectional shape (opening shape) of each recess 31 include a circular shape, an elliptical shape, a polygonal shape, etc. When the cross-sectional shape of each recess 31 is a circular shape, the diameter thereof is preferably 0.05 to 1.3 mm.

Due to such a configuration, even when a high current flows in quick temperature rise, it is possible to dissipate heat inside the resistor 3 to each lead 8 having a lower resistance value than that of the resistor 3. Therefore, it is possible to restrain heat from staying at the junction and to reduce load by heat generation.

In other words, since the inside of each recess 31 becomes the composition of the lead 8 having a lower resistance than that of the resistor 3, it is possible to reduce load by heat generation and reduce stress.

As a result, even when a high current flows in quick temperature rise, it is possible to restrain a crack from occurring in the junction. In addition, even when a current is repeatedly passed and the temperature is increased or decreased, it is possible to restrain a crack from occurring in the junction, and the reliability and the durability of the heater 1 are improved.

Here, in the heater 1 according to the embodiment, as shown in FIGS. 3 and 4, each recess 31 of the resistor 3 at the junction is preferably provided at the center of the end surface of the resistor 3. Thus, even when a high current flows and the resistor 3 rapidly generates heat in quick temperature rise, it is possible to substantially uniformly dissipate heat which is generated in the resistor 3 and is hard to dissipate, in an outer peripheral direction via the lead 8 within each recess 31. Thus, it is possible to reduce stress concentration, and thus it is possible to provide such a configuration that the product resistance is not changed even with long-term use.

It should be noted that the heater 1 shown in FIG. 3 has a shape in which each end portion of the resistor 3 is inserted into a substantially center portion of the front end portion of each lead 8 in a cross section. In the heater 1 shown in FIG. 4, each end portion of the resistor 3 is inserted into an inward portion of the front end portion of each lead 8 in a cross section, the distance from the resistor 3 to the surface of the heater 1 is long, and insulating properties are good. Thus, the shape shown in FIG. 4 is preferred.

In addition, as shown in FIGS. 5( a) and 5(b), it is preferred that there is no corner in the inner surface of the recess 31 of the resistor 3 at each junction. Since no acute corner is present in the inner surface of each recess 31, namely, the inner surface is a quadric surface, stress is not concentrated at each recess 31, and no crack occurs therein. As a result, the product resistance is not changed even with long-term use. Therefore, the reliability and the durability of the heater 1 are further improved. It should be noted that the heater 1 shown in FIG. 5( a) has a shape in which the recess 31 is provided on the substantially entirety of each end surface of the resistor 3, and the heater 1 shown in FIG. 5( b) has a shape in which the recess 31 is provided on only a substantially center portion of each end of the resistor 3. The shape shown in FIG. 5( a) is preferred in that it is possible to further reduce load by heat generation and effectively reduce stress.

In addition, it is preferred that the recess 31 of the resistor 3 at the junction is provided on both end surfaces of the resistor 3. Thus, regardless of the anode side and the cathode side, it is possible to reduce load by heat generation, and hence even when setting is performed with no concern for the anode side and the cathode side and long-term use is made, the product resistance is not changed. Therefore, it is possible to further improve the reliability and the durability of the heater 1.

It should be noted that the heaters 1 shown in FIGS. 1 to 5 have a shape in which each end portion of the resistor 3 is inserted into the front end portion of each lead 8 so as to be surrounded by the front end portion of each lead 8. In the heater according to the present invention, as long as each lead 8 has a shape thicker than the resistor 3 and is connected to the resistor 3 such that the end portion of the resistor 3 is inserted into the front end portion of the lead 8, each end portion of the resistor 3 may not be necessarily surrounded by the front end portion of the lead 8 over the entire circumference thereof and, for example, the front end portion of each lead 8 may have a cutout at a portion or a plurality of locations. However, preferably, each end portion of the resistor 3 is inserted into the front end portion of the lead 8 so as to be surrounded by the front end portion of the lead 8. Thus, in quick temperature rise, each lead 8 which covers the resistor 3 which thermally expands serves as a cushioning material for insulating ceramics having a different coefficient of linear expansion, and stress concentration is reduced. Thus, no crack occurs. As a result, the product resistance is not changed even with long-term use. Therefore, it is possible to further improve the reliability and the durability of the heater 1.

The heater 1 according to the embodiment is preferably used as a glow plug including the heater 1 and a metallic retaining member 7 which is electrically connected to the lead 8 and retains the heater 1, as shown in FIG. 6. The metallic retaining member 7 is a cylindrical body which retains the heater 1, and is joined to one of the leads 8 which is drawn out to the side surface of the ceramic base 9, by a solder material or the like. Thus, even when long-term use is made while ON/OFF is repeated in an engine at a high temperature, the resistance of the heater 1 is not changed. Therefore, it is possible to provide a glow plug which has good ignitability at any time.

Next, a method for manufacturing the heater 1 according to the embodiment will be described.

The heater 1 according to the embodiment may be formed by, for example, an injection molding method or the like using molds having the shapes of the resistor 3, each lead 8, and the insulating base 9.

First, a conductive paste which contains conductive ceramic powder, a resin binder, and the like and is to be the resistor 3 and each lead 8 is prepared, and a ceramic paste which contains insulating ceramic powder, a resin binder, and the like and is to be the insulating base 9 is prepared.

Next, a molded body of the conductive paste having a predetermined pattern which is to be the resistor 3 (a molded body A) is formed by an injection molding method or the like using the conductive paste. In a state where the molded body A is retained within a mold, the conductive paste is injected into the mold to form a molded body of the conductive paste having a predetermined pattern which is to be each lead 8 (a molded body B). Thus, a state is provided in which the molded body A and the molded body B connected thereto are retained within the mold.

Next, in the state where the molded body A and the molded body B are retained within the mold, a portion of the mold is replaced with a mold for molding the insulating base 9, and then the ceramic paste which is to be the insulating base 9 is injected into the mold. Thus, a molded body of the heater 1 (a molded body D) in which the molded body A and the molded body B are covered with a molded body of the ceramic paste (a molded body C) is obtained.

Next, the obtained molded body D is fired, for example, at a temperature of 1650° C. to 1800° C. under a pressure of 30 MPa to 50 MPa, whereby it is possible to produce the heater 1. The firing is preferably conducted in a non-oxidizing gas atmosphere such as hydrogen gas.

EXAMPLES

A heater according to an example of the present invention was produced as follows.

First, injection molding of a conductive paste containing 50% by mass of tungsten carbide (WC) powder, 35% by mass of silicon nitride (Si₃N₄) powder, and 15% by mass of a resin binder was conducted within a mold to produce a molded body A which is to be a resistor.

Next, in a state where the molded body A was retained within a mold, the above conductive paste which is to be each lead was injected into the mold to be connected to the molded body A, to form a molded body B which is to be each lead. At that time, a junction between the resistor and each lead was formed using molds having various shapes.

Next, in a state where the molded body A and the molded body B were retained within a mold, injection molding of a ceramic paste containing 85% by mass of silicon nitride (Si₃N₄) powder, 10% by mass of an oxide (Yb₂O₃) of ytterbium (Yb) as a sintering aid, and 5% by mass of WC for making a coefficient of thermal expansion to be close to those of the resistor and each lead was conducted within the mold. By so doing, a molded body D was formed which has a configuration in which the molded body A and the molded body B are buried in a molded body C which is to be an insulating base.

Next, the obtained molded body D was placed into a cylindrical mold made of carbon, and then sintered by conducting hot pressing at 1700° C. under a pressure of 35 MPa in a non-oxidizing gas atmosphere composed of nitrogen gas. A metallic retaining member was soldered to a lead end portion exposed on the surface of the obtained sintered body, to produce a heater.

Here, a heater in the form shown in FIG. 2 was produced as an example. At that time, a heater was produced in which the thickness of the resistor 3 in the up-down direction is 0.9 mm, the width thereof in the horizontal direction is 0.6 mm, each end portion of the resistor 3 is inserted into the front end portion of each lead 8 by 0.5 mm, the depth of the recess 31 provided on each end surface of the resistor 3 is 0.05 mm, and the diameter of each recess 31 is 0.5 mm.

In addition, as a comparative example, a heater was produced in which the thickness of the resistor 3 in the up-down direction is 0.9 mm, the width thereof in the horizontal direction is 0.6 mm, each end portion of the resistor 3 is not inserted into the front end portion of each lead 8, and no recess 31 is present on each end surface of the resistor 3.

A cooling/heating cycle test was conducted using these heaters. As the conditions of the cooling/heating cycle test, an applied voltage was set such that the temperature of the resistor became 1400° C. by passing a current through each heater, and 1) current passing for 5 minutes and 2) no current passing for 2 minutes were set as a single cycle, and this single cycle was repeated ten thousand times.

A change in the resistance value of each heater between before and after the cooling/heating cycle test was measured, and the change in the resistance of the sample according to the example of the present invention was equal to or lower than 1%. In addition, no trace of local heat generation was present at a connection portion between the resistor and each lead in this sample, and no micro crack was observed. In contrast, the change in the resistance of the sample according to the comparative example was equal to or higher than 5%, and a micro crack was observed.

REFERENCE SIGNS LIST

1 heater

3 resistor

31 recess

4 heat-generating portion

7 metallic retaining member

8 lead

9 insulating base 

1. A heater comprising: an insulating base; a resistor buried in the insulating base; and a lead buried in the insulating base, connected at a front end side thereof to the resistor, and drawn out at a rear end side thereof to a surface of the insulating base, wherein the lead comprises a shape thicker than the resistor and is connected to the resistor such that an end portion of the resistor is inserted into a front end portion of the lead, a recess is provided on an end surface of the resistor, and a portion of the lead is inserted into the recess.
 2. The heater according to claim 1, wherein the recess is provided at a center of the end surface of the resistor.
 3. The heater according to claim 1, wherein no corner is present in an inner surface of the recess.
 4. The heater according to claim 1, wherein the recess is provided on both end surfaces of the resistor.
 5. The heater according to claim 1, wherein the end portion of the resistor is inserted into the front end portion of the lead so as to be surrounded by the front end portion of the lead.
 6. A glow plug comprising: the heater according to claim 1; and a metallic retaining member which is electrically connected to the lead and retains the heater. 